一、前言
前面介绍了ServerCnxn，下面开始学习NIOServerCnxn。
二、NIOServerCnxn源码分析
2.1 类的继承关系
public class NIOServerCnxn extends ServerCnxn {}
说明：NIOServerCnxn继承了ServerCnxn抽象类，使用NIO来处理与客户端之间的通信，使用单线程处理。
2.2 类的内部类
1. SendBufferWriter类
private class SendBufferWriter extends Writer {
private StringBuffer sb = new StringBuffer();
/**
* Check if we are ready to send another chunk.
* @param force force sending, even if not a full chunk
*/
// 是否准备好发送另一块
private void checkFlush(boolean force) {
if ((force && sb.length() > 0) || sb.length() > 2048) { // 当强制发送并且sb大小大于0，或者sb大小大于2048即发送缓存
sendBufferSync(ByteBuffer.wrap(sb.toString().getBytes()));
// clear our internal buffer
sb.setLength(0);
}
}
@Override
public void close() throws IOException {
if (sb == null) return;
// 关闭之前需要强制性发送缓存
checkFlush(true);
sb = null; // clear out the ref to ensure no reuse
}
@Override
public void flush() throws IOException {
checkFlush(true);
}
@Override
public void write(char[] cbuf, int off, int len) throws IOException {
sb.append(cbuf, off, len);
checkFlush(false);
}
}
SendBufferWriter
说明：该类用来将给客户端的响应进行分块，其核心方法是checkFlush方法，其源码如下
private void checkFlush(boolean force) {
if ((force && sb.length() > 0) || sb.length() > 2048) { // 当强制发送并且sb大小大于0，或者sb大小大于2048即发送缓存
sendBufferSync(ByteBuffer.wrap(sb.toString().getBytes()));
// clear our internal buffer
sb.setLength(0);
}
}
说明：当需要强制发送时，sb缓冲中只要有内容就会同步发送，或者是当sb的大小超过2048（块）时就需要发送，其会调用NIOServerCnxn的sendBufferSync方法，该之后会进行分析，然后再清空sb缓冲。
2. CommandThread类
private abstract class CommandThread extends Thread {
PrintWriter pw;
CommandThread(PrintWriter pw) {
this.pw = pw;
}
public void run() {
try {
commandRun();
} catch (IOException ie) {
LOG.error("Error in running command ", ie);
} finally {
cleanupWriterSocket(pw);
}
}
public abstract void commandRun() throws IOException;
}
CommandThread
说明：该类用于处理ServerCnxn中的定义的命令，其主要逻辑定义在commandRun方法中，在子类中各自实现，这是一种典型的工厂方法，每个子类对应着一个命令，每个命令使用单独的线程进行处理。
2.3 类的属性
public class NIOServerCnxn extends ServerCnxn {
// 日志
static final Logger LOG = LoggerFactory.getLogger(NIOServerCnxn.class);
// ServerCnxn工厂
NIOServerCnxnFactory factory;
// 针对面向流的连接套接字的可选择通道
final SocketChannel sock;
// 表示 SelectableChannel 在 Selector 中注册的标记
private final SelectionKey sk;
// 初始化标志
boolean initialized;
// 分配四个字节缓冲区
ByteBuffer lenBuffer = ByteBuffer.allocate(4);
// 赋值incomingBuffer
ByteBuffer incomingBuffer = lenBuffer;
// 缓冲队列
LinkedBlockingQueue<ByteBuffer> outgoingBuffers = new LinkedBlockingQueue<ByteBuffer>();
// 会话超时时间
int sessionTimeout;
// ZooKeeper服务器
private final ZooKeeperServer zkServer;
/**
* The number of requests that have been submitted but not yet responded to.
*/
// 已经被提交但还未响应的请求数量
int outstandingRequests;
/**
* This is the id that uniquely identifies the session of a client. Once
* this session is no longer active, the ephemeral nodes will go away.
*/
// 会话ID
long sessionId;
// 下个会话ID
static long nextSessionId = 1;
int outstandingLimit = 1;
private static final String ZK_NOT_SERVING =
"This ZooKeeper instance is not currently serving requests";
private final static byte fourBytes[] = new byte[4];
}
类的属性
说明：NIOServerCnxn维护了服务器与客户端之间的Socket通道、用于存储传输内容的缓冲区、会话ID、ZooKeeper服务器等。
2.4 类的构造函数
public NIOServerCnxn(ZooKeeperServer zk, SocketChannel sock,
SelectionKey sk, NIOServerCnxnFactory factory) throws IOException {
this.zkServer = zk;
this.sock = sock;
this.sk = sk;
this.factory = factory;
if (this.factory.login != null) {
this.zooKeeperSaslServer = new ZooKeeperSaslServer(factory.login);
}
if (zk != null) {
outstandingLimit = zk.getGlobalOutstandingLimit();
}
sock.socket().setTcpNoDelay(true);
/* set socket linger to false, so that socket close does not
* block */
// 设置linger为false，以便在socket关闭时不会阻塞
sock.socket().setSoLinger(false, -1);
// 获取IP地址
InetAddress addr = ((InetSocketAddress) sock.socket()
.getRemoteSocketAddress()).getAddress();
// 认证信息中添加IP地址
authInfo.add(new Id("ip", addr.getHostAddress()));
// 设置感兴趣的操作类型
sk.interestOps(SelectionKey.OP_READ);
}
构造函数
说明：在构造函数中会对Socket通道进行相应设置，如设置TCP连接无延迟、获取客户端的IP地址并将此信息进行记录，方便后续认证，最后设置SelectionKey感兴趣的操作类型为READ。
2.5 核心函数分析
1. sendBuffer函数
public void sendBuffer(ByteBuffer bb) {
try {
if (bb != ServerCnxnFactory.closeConn) { // 不关闭连接
// We check if write interest here because if it is NOT set,
// nothing is queued, so we can try to send the buffer right
// away without waking up the selector
// 首先检查interestOps中是否存在WRITE操作，如果没有
// 则表示直接发送缓冲而不必先唤醒selector
if ((sk.interestOps() & SelectionKey.OP_WRITE) == 0) { // 不为write操作
try {
// 将缓冲写入socket
sock.write(bb);
} catch (IOException e) {
// we are just doing best effort right now
}
}
// if there is nothing left to send, we are done
if (bb.remaining() == 0) { // bb中的内容已经被全部读取
// 统计发送包信息（调用ServerCnxn方法）
packetSent();
return;
}
}
synchronized(this.factory){ // 同步块
// Causes the first selection operation that has not yet returned to return immediately
// 让第一个还没返回（阻塞）的selection操作马上返回结果
sk.selector().wakeup();
if (LOG.isTraceEnabled()) {
LOG.trace("Add a buffer to outgoingBuffers, sk " + sk
+ " is valid: " + sk.isValid());
}
// 将缓存添加至队列
outgoingBuffers.add(bb);
if (sk.isValid()) { // key是否合法
// 将写操作添加至感兴趣的集合
sk.interestOps(sk.interestOps() | SelectionKey.OP_WRITE);
}
}
} catch(Exception e) {
LOG.error("Unexpected Exception: ", e);
}
}
sendBuffer
说明：该函数将缓冲写入socket中，其大致处理可以分为两部分，首先会判断ByteBuffer是否为关闭连接的信号，并且当感兴趣的集合中没有写操作时，其会立刻将缓存写入socket，步骤如下
if (bb != ServerCnxnFactory.closeConn) { // 不关闭连接
// We check if write interest here because if it is NOT set,
// nothing is queued, so we can try to send the buffer right
// away without waking up the selector
// 首先检查interestOps中是否存在WRITE操作，如果没有
// 则表示直接发送缓冲而不必先唤醒selector
if ((sk.interestOps() & SelectionKey.OP_WRITE) == 0) { // 不为write操作
try {
// 将缓冲写入socket
sock.write(bb);
} catch (IOException e) {
// we are just doing best effort right now
}
}
// if there is nothing left to send, we are done
if (bb.remaining() == 0) { // bb中的内容已经被全部读取
// 统计发送包信息（调用ServerCnxn方法）
packetSent();
return;
}
}
当缓冲区被正常的写入到socket后，会直接返回，然而，当原本就对写操作感兴趣时，其会走如下流程
synchronized(this.factory){ // 同步块
// Causes the first selection operation that has not yet returned to return immediately
// 让第一个还没返回（阻塞）的selection操作马上返回结果
sk.selector().wakeup();
if (LOG.isTraceEnabled()) {
LOG.trace("Add a buffer to outgoingBuffers, sk " + sk
+ " is valid: " + sk.isValid());
}
// 将缓存添加至队列
outgoingBuffers.add(bb);
if (sk.isValid()) { // key是否合法
// 将写操作添加至感兴趣的集合
sk.interestOps(sk.interestOps() | SelectionKey.OP_WRITE);
}
}
首先会唤醒上个被阻塞的selection操作，然后将缓冲添加至outgoingBuffers队列中，后续再进行发送。
2. doIO函数
void doIO(SelectionKey k) throws InterruptedException {
try {
if (isSocketOpen() == false) { // socket未开启
LOG.warn("trying to do i/o on a null socket for session:0x"
+ Long.toHexString(sessionId));
return;
}
if (k.isReadable()) { // key可读
// 将内容从socket写入incoming缓冲
int rc = sock.read(incomingBuffer);
if (rc < 0) { // 流结束异常，无法从客户端读取数据
throw new EndOfStreamException(
"Unable to read additional data from client sessionid 0x"
+ Long.toHexString(sessionId)
+ ", likely client has closed socket");
}
if (incomingBuffer.remaining() == 0) { // 缓冲区已经写满
boolean isPayload;
// 读取下个请求
if (incomingBuffer == lenBuffer) { // start of next request
// 翻转缓冲区，可读
incomingBuffer.flip();
// 读取lenBuffer的前四个字节，当读取的是内容长度时则为true，否则为false
isPayload = readLength(k);
// 清除缓冲
incomingBuffer.clear();
} else { // 不等，因为在readLength中根据Len已经重新分配了incomingBuffer
// continuation
isPayload = true;
}
if (isPayload) { // 不为四个字母，为实际内容 // not the case for 4letterword
// 读取内容
readPayload();
}
else { // 四个字母，为四字母的命令
// four letter words take care
// need not do anything else
return;
}
}
}
if (k.isWritable()) { // key可写
// ZooLog.logTraceMessage(LOG,
// ZooLog.CLIENT_DATA_PACKET_TRACE_MASK
// "outgoingBuffers.size() = " +
// outgoingBuffers.size());
if (outgoingBuffers.size() > 0) {
// ZooLog.logTraceMessage(LOG,
// ZooLog.CLIENT_DATA_PACKET_TRACE_MASK,
// "sk " + k + " is valid: " +
// k.isValid());
/*
* This is going to reset the buffer position to 0 and the
* limit to the size of the buffer, so that we can fill it
* with data from the non-direct buffers that we need to
* send.
*/
// 分配的直接缓冲
ByteBuffer directBuffer = factory.directBuffer;
// 清除缓冲
directBuffer.clear();
for (ByteBuffer b : outgoingBuffers) { // 遍历
if (directBuffer.remaining() < b.remaining()) { // directBuffer的剩余空闲长度小于b的剩余空闲长度
/*
* When we call put later, if the directBuffer is to
* small to hold everything, nothing will be copied,
* so we've got to slice the buffer if it's too big.
*/
// 缩小缓冲至directBuffer的大小
b = (ByteBuffer) b.slice().limit(
directBuffer.remaining());
}
/*
* put() is going to modify the positions of both
* buffers, put we don't want to change the position of
* the source buffers (we'll do that after the send, if
* needed), so we save and reset the position after the
* copy
*/
// 记录b的当前position
int p = b.position();
// 将b写入directBuffer
directBuffer.put(b);
// 设置回b的原来的position
b.position(p);
if (directBuffer.remaining() == 0) { // 已经写满
break;
}
}
/*
* Do the flip: limit becomes position, position gets set to
* 0. This sets us up for the write.
*/
// 翻转缓冲区，可读
directBuffer.flip();
// 将directBuffer的内容写入socket
int sent = sock.write(directBuffer);
ByteBuffer bb;
// Remove the buffers that we have sent
while (outgoingBuffers.size() > 0) { // outgoingBuffers中还存在Buffer
// 取队首元素，但并不移出
bb = outgoingBuffers.peek();
if (bb == ServerCnxnFactory.closeConn) { // 关闭连接，抛出异常
throw new CloseRequestException("close requested");
}
// bb还剩余多少元素没有被发送
int left = bb.remaining() - sent;
if (left > 0) { // 存在元素未被发送
/*
* We only partially sent this buffer, so we update
* the position and exit the loop.
*/
// 更新bb的position
bb.position(bb.position() + sent);
break;
}
// 发送包，调用ServerCnxn方法
packetSent();
/* We've sent the whole buffer, so drop the buffer */
// 已经发送完buffer的所有内容，移除buffer
sent -= bb.remaining();
outgoingBuffers.remove();
}
// ZooLog.logTraceMessage(LOG,
// ZooLog.CLIENT_DATA_PACKET_TRACE_MASK, "after send,
// outgoingBuffers.size() = " + outgoingBuffers.size());
}
synchronized(this.factory){ // 同步块
if (outgoingBuffers.size() == 0) { // outgoingBuffers不存在buffer
if (!initialized
&& (sk.interestOps() & SelectionKey.OP_READ) == 0) { // 未初始化并且无读请求
throw new CloseRequestException("responded to info probe");
}
// 重置感兴趣的集合
sk.interestOps(sk.interestOps()
& (~SelectionKey.OP_WRITE));
} else { // 重置感兴趣的集合
sk.interestOps(sk.interestOps()
| SelectionKey.OP_WRITE);
}
}
}
} catch (CancelledKeyException e) {
LOG.warn("Exception causing close of session 0x"
+ Long.toHexString(sessionId)
+ " due to " + e);
if (LOG.isDebugEnabled()) {
LOG.debug("CancelledKeyException stack trace", e);
}
close();
} catch (CloseRequestException e) {
// expecting close to log session closure
close();
} catch (EndOfStreamException e) {
LOG.warn("caught end of stream exception",e); // tell user why
// expecting close to log session closure
close();
} catch (IOException e) {
LOG.warn("Exception causing close of session 0x"
+ Long.toHexString(sessionId)
+ " due to " + e);
if (LOG.isDebugEnabled()) {
LOG.debug("IOException stack trace", e);
}
close();
}
}
doIO
说明：该函数主要是进行IO处理，当传入的SelectionKey是可读时，其处理流程如下
if (k.isReadable()) { // key可读
// 将内容从socket写入incoming缓冲
int rc = sock.read(incomingBuffer);
if (rc < 0) { // 流结束异常，无法从客户端读取数据
throw new EndOfStreamException(
"Unable to read additional data from client sessionid 0x"
+ Long.toHexString(sessionId)
+ ", likely client has closed socket");
}
if (incomingBuffer.remaining() == 0) { // 缓冲区已经写满
boolean isPayload;
// 读取下个请求
if (incomingBuffer == lenBuffer) { // start of next request
// 翻转缓冲区，可读
incomingBuffer.flip();
// 读取lenBuffer的前四个字节，当读取的是内容长度时则为true，否则为false
isPayload = readLength(k);
// 清除缓冲
incomingBuffer.clear();
} else { // 不等，因为在readLength中根据Len已经重新分配了incomingBuffer
// continuation
isPayload = true;
}
if (isPayload) { // 不为四个字母，为实际内容 // not the case for 4letterword
// 读取内容
readPayload();
}
else { // 四个字母，为四字母的命令
// four letter words take care
// need not do anything else
return;
}
}
}
说明：首先从socket中将数据读入incomingBuffer中，再判断incomingBuffer是否与lenBuffer相等，若相等，则表示读取的是一个四个字母的命令，否则表示读取的是具体内容的长度，因为在readLength函数会根据socket中内容的长度重新分配incomingBuffer。其中，readLength函数的源码如下
private boolean readLength(SelectionKey k) throws IOException {
// Read the length, now get the buffer
// 读取position之后的四个字节
int len = lenBuffer.getInt();
if (!initialized && checkFourLetterWord(sk, len)) { // 未初始化并且是四个字母组成的命令
return false;
}
if (len < 0 || len > BinaryInputArchive.maxBuffer) {
throw new IOException("Len error " + len);
}
if (zkServer == null) {
throw new IOException("ZooKeeperServer not running");
}
// 重新分配len长度的缓冲
incomingBuffer = ByteBuffer.allocate(len);
return true;
}
说明：首先会读取lenBuffer缓冲的position之后的四个字节，然后判断其是否是四字母的命令或者是长整形（具体内容的长度），之后再根据长度重新分配incomingBuffer大小。
同时，在调用完readLength后，会知道是否为内容，若为内容，则会调用readPayload函数来读取内容，其源码如下
private void readPayload() throws IOException, InterruptedException {
// 表示还未读取完socket中内容
if (incomingBuffer.remaining() != 0) { // have we read length bytes?
// 将socket的内容读入缓冲
int rc = sock.read(incomingBuffer); // sock is non-blocking, so ok
if (rc < 0) { // 流结束异常，无法从客户端读取数据
throw new EndOfStreamException(
"Unable to read additional data from client sessionid 0x"
+ Long.toHexString(sessionId)
+ ", likely client has closed socket");
}
}
// 表示已经读取完了Socket中内容
if (incomingBuffer.remaining() == 0) { // have we read length bytes?
// 接收到packet
packetReceived();
// 翻转缓冲区
incomingBuffer.flip();
if (!initialized) { // 未初始化
// 读取连接请求
readConnectRequest();
} else {
// 读取请求
readRequest();
}
// 清除缓冲
lenBuffer.clear();
// 赋值incomingBuffer，即清除incoming缓冲
incomingBuffer = lenBuffer;
}
}
说明：首先会将socket中的实际内容写入incomingBuffer中（已经重新分配大小），当读取完成后，则更新接收的包统计信息，之后再根据是否初始化了还确定读取连接请求还是直接请求，最后会清除缓存，并重新让incomingBuffer与lenBuffer相等，表示该读取过程结束。
而当doIO中的key为可写时，其处理流程如下
if (k.isWritable()) { // key可写
// ZooLog.logTraceMessage(LOG,
// ZooLog.CLIENT_DATA_PACKET_TRACE_MASK
// "outgoingBuffers.size() = " +
// outgoingBuffers.size());
if (outgoingBuffers.size() > 0) {
// ZooLog.logTraceMessage(LOG,
// ZooLog.CLIENT_DATA_PACKET_TRACE_MASK,
// "sk " + k + " is valid: " +
// k.isValid());
/*
* This is going to reset the buffer position to 0 and the
* limit to the size of the buffer, so that we can fill it
* with data from the non-direct buffers that we need to
* send.
*/
// 分配的直接缓冲
ByteBuffer directBuffer = factory.directBuffer;
// 清除缓冲
directBuffer.clear();
for (ByteBuffer b : outgoingBuffers) { // 遍历
if (directBuffer.remaining() < b.remaining()) { // directBuffer的剩余空闲长度小于b的剩余空闲长度
/*
* When we call put later, if the directBuffer is to
* small to hold everything, nothing will be copied,
* so we've got to slice the buffer if it's too big.
*/
// 缩小缓冲至directBuffer的大小
b = (ByteBuffer) b.slice().limit(
directBuffer.remaining());
}
/*
* put() is going to modify the positions of both
* buffers, put we don't want to change the position of
* the source buffers (we'll do that after the send, if
* needed), so we save and reset the position after the
* copy
*/
// 记录b的当前position
int p = b.position();
// 将b写入directBuffer
directBuffer.put(b);
// 设置回b的原来的position
b.position(p);
if (directBuffer.remaining() == 0) { // 已经写满
break;
}
}
/*
* Do the flip: limit becomes position, position gets set to
* 0. This sets us up for the write.
*/
// 翻转缓冲区，可读
directBuffer.flip();
// 将directBuffer的内容写入socket
int sent = sock.write(directBuffer);
ByteBuffer bb;
// Remove the buffers that we have sent
while (outgoingBuffers.size() > 0) { // outgoingBuffers中还存在Buffer
// 取队首元素，但并不移出
bb = outgoingBuffers.peek();
if (bb == ServerCnxnFactory.closeConn) { // 关闭连接，抛出异常
throw new CloseRequestException("close requested");
}
// bb还剩余多少元素没有被发送
int left = bb.remaining() - sent;
if (left > 0) { // 存在元素未被发送
/*
* We only partially sent this buffer, so we update
* the position and exit the loop.
*/
// 更新bb的position
bb.position(bb.position() + sent);
break;
}
// 发送包，调用ServerCnxn方法
packetSent();
/* We've sent the whole buffer, so drop the buffer */
// 已经发送完buffer的所有内容，移除buffer
sent -= bb.remaining();
outgoingBuffers.remove();
}
// ZooLog.logTraceMessage(LOG,
// ZooLog.CLIENT_DATA_PACKET_TRACE_MASK, "after send,
// outgoingBuffers.size() = " + outgoingBuffers.size());
}
说明：其首先会判断outgoingBuffers中是否还有Buffer未发送，然后遍历Buffer，为提供IO效率，借助了directBuffer（64K大小），之后每次以directBuffer的大小(64K)来将缓冲的内容写入socket中发送，直至全部发送完成。
三、总结
本篇讲解了NIOServerCnxn的处理细节，其主要依托于Java的NIO相关接口来完成IO操作，也谢谢各位园友的观看~